Laser beams provide a means of writing, bar coding, and decoratively marking substrates. Advantages of the use of lasers over conventional printing technologies include the ease with which layouts can be adjusted and integrated into production lines using computer graphics programs. Laser marking enables a contact-free procedure, even on soft, irregular surfaces that are not readily accessible. In addition, laser marking is ink-free, which makes it long lasting. It is also solvent-free, and thus environmentally advantageous.
Color imaging with a laser beam can be achieved through the use of leuco dyes and sensitizers or through the use of appropriate pigments. For instance, U.S. Pat. No. 4,307,047 describes the use of iron oxide hydroxide that yields a color when water of crystallization of the oxide is split off at 260° C. U.S. Pat. No. 6,214,916 describes a resin composition having laser marking properties employing a Neodymium-Doped Yttrium-Aluminum-Garnet (“NdYAG”) radiation laser on a composition comprising a polyester thermoplastic resin, an amount of light pigment sufficient to form a light background coloration, and an effective amount of marking agent. The polyester thermoplastic resin decomposes in areas struck by the laser to form dark colored markings on the light background coloration. Disclosed marking agents are boron phosphate, zinc oxide, zinc stannate, zinc hydrostannate, tin (II) oxalate, and mixtures thereof.
Several laser types are available for marking surfaces. Excimer lasers with frequencies in the range 196-351 nanometers lead to the marking of surfaces by photochemical ablation or reaction. Using NdYAG lasers at lower power levels at 532 nanometers provides laser marking by leaching or selective bleaching of dyes or pigments. Using NdYAG lasers at 1064 nanometers leads to laser marking by carbonization, sublimation, discoloration, foaming, or engraving. Use of CO2 lasers at 10600 nanometers enables laser marking by thermochemical reaction, melting, vaporizing, and engraving. Speeds of up to 10,000 mm/sec are possible with CO2 lasers, while NdYAG lasers allow speeds of up to 2000 mm/sec.
Several materials have been found to be useful for providing contrast in laser marking. One type of laser marking provides a light contrast on a dark background. Carbon black may be used in this approach. Carbon black works by decomposing into volatile components after absorbing laser light. The volatile components foam at the surface of the substrate incorporating the carbon black, leading to light scattering and thus a light impression. EP 0 675 001 teaches that zinc borate, which releases its water of hydration, may also be used as a contrast-enhancing additive. U.S. Pat. No. 4,595,647 discloses a laser-markable material useful for encapsulation of electronic devices. In this system, TiO2 or TiO2 and CrO3 are added to common plastic encapsulants formed from a mixture of a resin/filler/carbon black/mold release agent. When irradiated by a CO2 laser, the originally grey material turns bright gold, providing a high contrast, durable mark. U.S. Pat. No. 5,063,137 teaches that anhydrous metal borate or metal phosphate salts, phosphoric acid-containing glass, basic zinc carbonate, and basic magnesium carbonate when mixed with a resin give, upon exposure to a laser, a white marking on a dark background.
A dark contrast on a light background is also possible using lasers. EP 0 111 357 and U.S. Pat. No. 4,578,329 disclose that metal silicates provide black markings on articles having a polyolefin surface. U.S. Pat. No. 5,489,639 teaches that copper phosphate, copper sulfate, and copper thiocyanate with a thermoplastic resin give dark markings upon treatment with a laser. U.S. Pat. No. 4,816,374 teaches that lead iodide, lead carbonate, lead sulfide, dioxin isocyanate, antimony, and related compounds and mixtures give dark markings upon treatment with a laser when used with polyolefin substrates.
The effects of different silicates on the laser marking of polyolefins are described in Kilp, “Laser Marking of Plastics”, Annu. Tech. Conf. Soc. Plast. Eng., 49th, pages 1901-1903 (1991). Kaolin gives white marks on colored substrates, while black marks are obtained when mica or titanium dioxide is incorporated into the substrate.
US 2002/0002225 describes black thermoplastic molding compositions which contain dye combinations made from non-absorbing, non-black polymer-soluble dyes that produce black thermoplastic molding compositions which are transparent or translucent to laser light. These compositions are used to laser-weld one thermoplastic resin to another. This published application focuses on anthraquinone dyes used in combination(s) to yield a black image.
Laser imaging in general is known to some degree. For instance, US 2002/0122931 is entitled “Papers and Cardboard Products Suitable for Laser Marking, Method for Producing Same and their Use for Packaging Materials, Bank Notes and Securities, Security Paper and Graphic Products.” This application relies on paperboard or paper with plate-like materials. US 2007/0148393 teaches at paragraphs [0072]-[0080] the use of a varnish with oxyanion metals for purposes of laser imaging. EP 0 190 997 B1 claims a method for the inscription of high molecular weight organic material which contains a radiation-sensitive additive which effects a change in color, where the radiated energy is directed onto the surface of the high molecular weight organic material. Laser light of specified wavelengths is taught to be useful. The additive contained in the high molecular weight organic material is taught to be an inorganic pigment and/or an organic pigment and/or a polymer-soluble dye.
Coumarin-type dyes are often employed in fillers in molding materials and plastic articles. The poor solubility of such dyestuffs in water has resulted in a perception that they are unsuitable as a water-based paper coating material.